NAME

       v.what.rast  - Uploads raster values at positions of vector points to the table.

KEYWORDS

       vector, sampling, raster, position, querying, attribute table, surface information

SYNOPSIS

       v.what.rast
       v.what.rast --help
       v.what.rast   [-ip]  map=name   [layer=string]    [type=string[,string,...]]   raster=name
       [column=name]   [where=sql_query]   [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -i
           Interpolate values from the nearest four cells

       -p
           Print categories and values instead of updating the database

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       map=name [required]
           Name of vector points map for which to edit attributes
           Or data source for direct OGR access

       layer=string
           Layer number or name
           Vector features can have category values in different layers. This  number  determines
           which layer to use. When used with direct OGR access this is the layer name.
           Default: 1

       type=string[,string,...]
           Input feature type
           Options: point, centroid
           Default: point

       raster=name [required]
           Name of existing raster map to be queried

       column=name
           Name of attribute column to be updated with the query result

       where=sql_query
           WHERE conditions of SQL statement without ’where’ keyword
           Example: income < 1000 and population >= 10000

DESCRIPTION

       v.what.rast  retrieves  raster  value  from  a given raster map for each point or centroid
       stored in a given vector map. It can update a column in the linked vector attribute  table
       with the retrieved raster cell value or print it.

       The  column  type needs to be numeric (integer, float, double, ...). If the column doesn’t
       exist in the vector attribute table than the module will create the  new  column  of  type
       corresponding with the input raster map.

       If  the  -p  flag  is  used,  then  the attribute table is not updated and the results are
       printed to standard output.

       If the -i flag is used, then the value to be uploaded to the database is interpolated from
       the  four  nearest  raster  cells values using an inverse distance weighting method (IDW).
       This is useful for cases when the vector point density is much higher than the raster cell
       size.

NOTES

       Points  and  centroid  with  shared  category number cannot be processed.  To solved this,
       unique categories may be added with v.category in a separate layer.

       If multiple points have the same category, the attribute value is set  to  NULL.   If  the
       raster value is NULL, then attribute value is set to NULL.

       v.what.rast  operates  on  the attribute table. To modify the vector geometry instead, use
       v.drape.

       Categories and values are output unsorted with the print  flag.  To  sort  them  pipe  the
       output  of  this  module into the UNIX sort tool (sort -n). If you need coordinates, after
       sorting use v.out.ascii and the UNIX paste tool (paste -d’|’).  In  the  case  of  a  NULL
       result, a "*" will be printed in lieu of the value.

       The  interpolation  flag is only useful for continuous value raster maps, if a categorical
       raster is given as input the results will be nonsense.  Since the search window is limited
       to four raster cells there may still be raster cell-edge artifacts visible in the results,
       this compromise has been made for processing speed. If one or more  of  the  nearest  four
       raster  cells  is  NULL,  then only the raster cells containing values will be used in the
       weighted average.

EXAMPLES

   Transferring raster values into existing attribute table of vector points map
       Reading values from raster map at position of vector points, writing these values  into  a
       column of the attribute table connected to the vector map:

       # work on copy of original geodetic points map
       g.copy vector=geodetic_pts,mygeodetic_pts
       # set computational region to raster map to be queried
       g.region raster=elev_state_500m -p
       # query raster cells (a new column will be added to existing table)
       v.what.rast map=mygeodetic_pts raster=elev_state_500m column=height
       # compare official geodetic heights to those of elevation model
       v.db.select map=mygeodetic_pts columns=Z_VALUE,height separator=comma

   Transferring raster values into new vector points map
       In case of a vector map without attached attribute table, first add a new attribute table.
       This table is then populated with values queried from the raster map:

       # create new random vector points map
       v.random pnts n=100
       # add new table, link to map
       v.db.addtable map=pnts column="height double precision"
       # set computational region to raster map to be queried
       g.region raster=elevation -p
       # query raster map and upload values to vector table into specified column
       v.what.rast map=pnts raster=elevation column=height
       # verify new attribute table:
       v.db.select pnts
       # verify statistics of uploaded values:
       v.univar map=pnts column=height type=point

SEE ALSO

        v.category, v.db.addtable, v.db.select, v.drape, v.univar, v.rast.stats, v.what.vect

AUTHORS

       Radim Blazek
       Hamish Bowman (interpolation)

SOURCE CODE

       Available at: v.what.rast source code (history)

       Accessed: Tuesday Jun 27 11:14:06 2023

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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual